With the progress in the scientific technological fields and in the modularization and miniaturization of camera lens, it is now possible to produce digital cameras having a very small volume, and most of currently available mobile phones are now provided with the function of a digital camera. There are many different auto-focus actuating mechanisms available for a miniature lens. Among others, the voice coil motor (VCM) is the currently most widely employed auto-focus actuating structure. The VCM has the advantages of small volume, low power consumption, accurately actuated displacement, and cost-effective, and is therefore very suitable for short-distance actuation in miniature lens auto-focusing.
FIG. 1 is an exploded perspective view of a conventional VCM auto-focusing structure 10, and FIG. 2 is a fragmentary, enlarged perspective view of the conventional VCM auto-focusing structure 10 of FIG. 1. As can be seen from FIGS. 1 and 2, the VCM auto-focusing structure 10 holds and moves a lens assembly 100 forward and rearward, so that the lens assembly 100 focuses the image of an object being shot on a light sensor device (not shown). The conventional VCM auto-focusing structure 10 mainly includes a housing 11 and a bottom cover 12. The housing 11 is internally provided with a lens holding mechanism 20 for holding and moving the lens assembly 100 to perform auto focusing. In the lens holding mechanism 20, an upper and a lower spring plate 13 are used to connect to an inner top of the housing 11 and to an inner surface of the bottom cover 12, respectively. The lens holding mechanism 20 further includes a lens holder 21 wounded around by a coil 22, and four magnets 14 provided in the housing 11 on four inner wall surfaces thereof and fixed in place via a magnet holding frame 15. When the coil 22 is supplied with electric current, a magnetic field is emitted at the coil 22 to magnetically attract or repulse the magnets 14, so that the lens holder 21 is pushed forward or rearward to achieve the purpose of auto focusing.
As can be seen from FIG. 2, the bottom cover 12 for the conventional VCM auto-focusing structure 10 has a stepped inner surface. The lower spring plate 13 is connected at an outer fixed portion 13′ thereof to the inner surface of the bottom cover 12, and at an inner movable portion 13″ thereof to a bottom surface of the lens holder 21. There is a distance L in an axial direction between the fixed portion 13′ and the movable portion 13″ of the lower spring plate 13. This is because the lens holding mechanism 20 utilizes an elastic pull force of the movable portion 13″ to pull the lens holder 21 closer to the inner surface of the bottom cover 12, so that the movable portion 13″ is pulled away from the fixed portion 13′ of the lower spring plate 13 by the distance L. Due to the distance L existing between the fixed and the movable portion 13′, 13″, the lower spring plate 13 generates a pre-tension, which necessitates more electric power to offset the pre-tension of the lower spring plate 13 before the lens holding mechanism 20 can be actuated. FIG. 3 shows a characteristic curve of the actuating current for actuating the conventional VCM auto-focusing structure 10. As shown, a relatively high amount of current about 20-30 mA is required to actuate the VCM auto-focusing structure 10. This is why the conventional VCM auto-focusing structure 10 needs four magnets to induce greater magnetic attraction and repulsion relative to the electromagnetic force of the coil 22 to actuate the auto focusing.
FIG. 4 shows the lens tilt characteristic of the conventional VCM auto-focusing structure at actuation thereof. Since the lens holder 21 is in contact with the inner surface of the bottom cover 12, and since the lower spring plate 13 might provide non-uniform elastic pull force due to insufficient flat contact between it and the bottom cover 12 or the housing 11, the lens holder 21 tends to tilt by an angle of about 20˜23 minutes (1 degree=60 minutes) at the instant the lens holding mechanism 20 is actuated to perform auto focusing. The lens tilt would adversely reduce the usable resolution of the lens. For example, in the event of a 20-minute lens tilt angle, the lens can only provide a resolution of 5 million pixels. However, when the lens tilt angle is reduced to 5 minutes, a lens resolution of 8 million pixels can be obtained.